1. Field of the Invention
The present invention relates to an image projecting apparatus. More particularly, the present invention relates to an image projecting apparatus that projects an image displayed on an image display element onto a screen at an enlarged scale using a projection lens (or projection optical system).
2. Description of the Related Art
Image projecting apparatuses that project an image of an image display element (such as a liquid crystal panel) onto a screen through a projection lens to provide an image are widely used. Assembly of lenses and lens units each composed of a plurality of lenses that constitute a projection lens into a lens barrel involves assembly errors, which will generate parallel decentration. In consequence, color shift occurs in projected images.
If large color shift occurs in the projection lens, images of three colors (red, green and blue) projected on the screen will be separated. This leads to a significant deterioration in the resolution (or image quality). Such deterioration in the image quality due to color shift also occurs similarly in an image pickup lens used in a camera that takes an image of an object. In connection with color shift, U.S. Pat. No. 6,038,068 discloses an optical system that is designed to reduce color shift caused by air dispersion. Further, Japanese Patent Application Laid-Open No. 2005-274837 discloses a zoom lens that has a decentration adjusting function to reduce effects of assembly errors on aberrations in an optical system composed of a plurality of lenses or lens units.
If an assembly error occurs upon assembling a plurality of lenses or lens units that constitute an optical system into a lens barrel, decentering aberrations are generated, and in particular large decentering lateral chromatic aberration is generated. The decentering lateral chromatic aberration refers to color shift in the direction perpendicular to the optical axis on the optical axis due to parallel decentration caused by an assembly error. The decentering lateral chromatic aberration leads to color shift in projected images in the case of projection lenses and color shift in images in the case of image pickup lenses, which causes a significant deterioration in the image quality. This problem has become more serious in recent years with an increase in the resolution of image display elements. It is very difficult to assemble a plurality of lenses and lens units in a lens barrel without assembly errors. Thus, it is a very effective method to decenter a lens(es) or a lens unit(s) which compose an optical system thereby reducing deterioration in the optical performance due to the assembly errors. However, it is still difficult to reduce color shift effectively by simple decentration adjustment of an optical element(s). In order to reduce color shift, it is important to appropriately select the optical elements in the optical system for which decentration adjustment is to be performed and to design the optical elements appropriately.